Oxygen demand is the rate of dissolved oxygen consumed in a fluid sample 12 over a given time at a given temperature. Biological oxygen demand (BOD) is the rate of the oxygen consumption specifically by aerobic organisms. Aerobic organisms in aquatic ecosystems or drinking water require dissolved oxygen to break apart organic materials. BOD is therefore a predictive value for readily decomposable organic molecules, or organic pollution, in a fluid (e.g. a wastewater effluent). BOD depends and varies based on temperature, nutrient concentrations, enzymes and microorganisms present in the sample. BOD is a calculated value and is not measured directly. Photosynthesis and respiration are arguably the defining parameters of carbon cycling in aquatic ecosystems (freshwater and saltwater). Respiration is also primary components of oxygen demand measurements (BOD) in the water quality community.
The Clean Water Act section 304(a)(4) designated BOD as a “conventional” pollutant. The Code of Federal Regulations Title 40, Chapter 1, Subchapter D, Part 136.3 (40 CFR Ch. I(d) § 136.3) establishes the guidelines and lists the approved test procedures for measuring substances, including BOD. The approved test is Standard Methods 5210 B; 5-day BOD Test (also referred to as BOD5), available at www.EPA.gov, and incorporated herein.
Alternative tests, that are much easier to perform, such as total organic carbon (TOC) and chemical oxygen demand (COD) are available but are less favored by regulators as BOD directly measures the bioavailability of the waste in the discharge. However the BOD test has limitations including the length of time required to run, results are imprecise and unpredictable, and the method requires skill and control of test conditions. In addition to the uncertainty factor, five days is problematic as it will be at least five days until the operator knows that a test was invalid and the opportunity to retest likely would have passed. Temperature control is a critical factor and would be more susceptible to swings over a longer period.
BOD tests provide an empirically calculated measure of the remaining oxygen demand in untreated wastewater or a treatment plant effluent. Whereas effluent limits are typically met during apparent normal operations of a biological treatment system, such is not assured and operators must wait five days for confirmation. Given the uncertainty created by the five day wait, facility operators may operate in a conservative mode and over-aerate the biological reactors. A need exists to obtain quicker BOD results to facilitate sufficient and efficient treatment.
Wastewater facilities generally practice aeration with set dissolved oxygen (DO) targets and control these levels without consideration of the actual moment-to-moment oxygen load requirement present in a tank. The over-aeration is typically an additional 20% to 40% more oxygen than actually needed for efficient processing. As a result, sludge aeration energy costs account for between 50% and 70% of facility operation costs. Measuring BOD at intervals significantly less than 5 days could facilitate vastly faster feedback control time of aeration levels based on BOD and could lead to at least a 3% to 6% reduction in overall plant costs.
The standard procedure for performing the 5-day test is detailed in the Standard Methods for Examination of Water and Wastewater (5210 Biochemical Oxygen Demand provided by the EPA). Ideally, samples should be processed within 2 hours of sample collection or should be otherwise stored at or below 4° C.±2° C. up to 24 hours post-collection but preferably less than 6 hours post-collection. The overall method consists of overfilling an airtight bottle with the fluid sample, diluting the sample, adding the chemical reagents, bringing dilution water temperature to 20±3° C., measuring the initial dissolved oxygen level, incubating the sample at 20±1° C. and measuring the final dissolved oxygen level 5 days post-collection. Significant opportunity exists to obtain invalid results and given the 5 day test period, no information, useful or not, is generated until the fifth day.
TABLE 1Summary of BOD quality control procedures.Quality Control ElementFrequencyAcceptance CriteriaUnseeded dilution waterOne per batch orDO uptake <0.2 mg/Lsample group(preferably <0.1 mg/L)(1 per 20 samplesminimum)Standard check solutionOne per batch orLaboratory controlsample grouplimits(1 per 20 samplesminimum)Seed control standardOne per batch orDO uptake betweensample group0.6 and 1.0 mg/L(1 per 20 sampleminimum)Duplicate sampleOne per batch orRPD 5X CRDL; ±CRDLsample groupfor samples(1 per 20 samplesminimum)Source: Biochemical Oxygen Demand (BOD) Standard Method 5210 B (5-day BOD Test); www.EPA.gov
While the BOD5 Standard Method meets the water quality testing requirements, the lag between sampling and data acquisition prevents effective use of the information for the benefit of improved operational efficiency. A five day compliance sample cannot provide timely data as it would be necessary to break the seal in advance of the 5 day wait period, invalidating the result. Sequential shortened-time results could be obtained by pulling multiple samples and testing at varying intervals. Unfortunately this would require many samples, further complicating maintenance of control of critical Standard Method parameters such as temperature and light. Thus, there is an unmet need for a continuous measurement of BOD with the ability for results reflecting the current state of the wastewater treatment process.